refactor: smaller cleanup tasks for casl refactor - #2759

## Description
Subsection of PR #2748 for datasets. PR depends on #2759 to be merged
first.

This unifies the datasetEndpointAccess and datasetInstanceAccess
functions in CaslAbilityFactory, removes instance-level Action elements
and adjusts all affected controllers to accommodate the change. The
dataset-specific code is extracted into a separate module

## Fixes

* The endpoints /proposals/:id/datasets and /samples/:id/datasets would
not apply correct filters due to a mismatch between the used casl
ability and checked Action elements

## Changes:

* Replace CaslAbilityFactory.datasetsInstanceAccess and
CaslAbilityFactory.datasetsEndpointAccess with one function
CaslAbilityFactory.datasetAccess
* Code for CaslAbilityFactory.datasetAccess is factored out into new
module DatasetAbility
* Remove all instance-level dataset Action elements
* Adjust endpoint and instance auth logic in dataset controllers
* Remove unused readOwner checks, treat readPublic as always true
(datasets-access.service)
* Adjust auth logic in samples, proposals, users and origdatablocks
controllers where dataset abilities are used

## Tests included

- [x] Included for each change/fix?
- [x] Passing? <!-- Merge will not be approved unless tests pass -->

## Documentation
- [x] swagger documentation updated (required for API changes)
- [x] official documentation updated

### official documentation info
<!-- If you have updated the official documentation, please provide PR #
and URL of the updated pages -->

## Summary by Sourcery

Consolidate dataset authorization into a single CASL ability and align
controllers and access services with the simplified permission model.

Bug Fixes:
- Ensure /proposals/:id/datasets, /samples/:id/datasets, and other
dataset-related endpoints apply correct filters based on unified dataset
permissions for authenticated and unauthenticated users.

Enhancements:
- Replace separate dataset endpoint and instance access factories with a
single datasetAccess ability using unified actions and conditions.
- Simplify dataset-related controllers, V4 controllers, and access
services to rely on generic Dataset* actions instead of
owner/access/public-specific variants.
- Remove numerous instance-level dataset Action enum values in favor of
a smaller, clearer action set, adding AccessAny for elevated roles.
- Treat public dataset reads consistently by defaulting unauthenticated
access to published datasets and tightening v4 behavior to require
public endpoints for anonymous users.
- Update Swagger-guarded policies and related controllers (datasets,
samples, proposals, origdatablocks, users) to use the new datasetAccess
checks.

Tests:
- Adjust dataset controller specs to mock the new datasetAccess factory
method and keep authorization tests passing.